Hi Dan,
I think the simple answer is "No, OWL is not 'able' to use XSD types
by reference using component designators." More precisely, the OWL 2
spec does not set out conditions for using XSD types by reference.
The OWL 2 type system is extensible by design (and in a much more
precise way than OWL 1 or RDF was...i.e., we have a very clear
description of how new types can be added and the effects on
ontologies), so, in principle, implementations could support this.
Originally, we intended for there to be two mechanisms, internal and
external, for adding use defined types. However, over time, in part
due to the slowness of progress on component designators and, more
seriously, some issues with the Schema types, we moved to "internal
only", i.e., we've only speced how to add user defined types from
within owl using the constructors we've specified (which are, as far
as sensible, aligned with XSD).
One additional barrier was the effort to define the constraints on
sensible user derived types that must be handled by conforming
implementations. This includes the fact that no owl system has any
prospects of handling complex types. One *could* do something along
these lines by defining a restriction on the XSD schema that made it
isomorphic to the syntactic restrictions in OWL 2. You would still
have some issues about the interpretation of some of the datatypes.
At this point, the working group (mostly because I gave up :))
decided that the demand for this feature, esp. given the internal
facilities, didn't warrant the effort at this time. It's also a
fairly modular piece of work, so if demand opens up, it wouldn't need
a revision to the core spec to add this feature.
(This is me speaking, not on behalf of the working group.)
Cheers,
Bijan.